Stainless Steel Grit: A Viable Alternative to Mineral Abrasive
Mineral abrasives, such as aluminum oxide and garnet, are popular with traditional users for general cleaning and in applications that require a specific surface roughness. Though aware of metallic options, these users tend to resist change and stick with the familiar.
Progressive end users who have tried steel abrasives are likely to report benefits such as increased recyclability, productivity and savings in operating costs. With the escalating costs of mineral abrasives, transportation and disposal, more customers are looking at steel abrasives for their cleaning operations.
Steel abrasives such as conventional shot and grit work well for most cleaning applications, but their inherent chemistry stops short of addressing transfer corrosion, an aspect that’s critical when transporting automobiles and other vehicles. Conventional steel shot and grit abrasive retained in the crevices of a blasted component will rust when exposed to commonly encountered corrosive environments. Transference to the surrounding space then becomes a risk. If, for example, that space is occupied by a new car in transit to the dealership, the exposed auto parts are subject to this corrosion. Therefore, there was a clear need to identify abrasive options that provide superior recyclability and the ability to mitigate corrosion risks.
Aluminum Oxide and its Use in Grit Blasting
Aluminum oxide (AlOx) has desirable properties that make it a popular choice for grit blasting. Its angular shape and hardness (9 on the Mohs scale) easily impart an etch on the blasted surface. However, the very same desirable characteristics also make it brittle, causing early disintegration and reduction in durability.
An acknowledged advantage of AlOx is its propensity to maintain its angular profile even after repeated impacts and breakdowns. At some point, the breakdown leads to angular but smaller-sized particles that no longer possess the initial energy to impart the intended surface roughness. In critical applications, where maintenance of a consistent profile is critical, a vibratory classifier separates the smaller-sized angular AlOx from the work mix.
The breakdown of this abrasive not only requires the system to be constantly replenished but also creates a dusty environment for the operators to work in. The dust in turn gets ventilated to a dust collector, where the user has to contend with the inconvenience of disposing high volumes of dust and fines. Other intangibles, leading to a drop in productivity, include operator discomfort, vision issues in a perennially clouded and dusty blast environment and repeated maintenance for all wear components that come in direct contact with the abrasive.
Use of Amagrit results in an operating environment with minimal dust (top), while using mineral abrasives such as AlOx results in a dusty room (bottom). PHOTOS COURTESY OF ervin industries, inc.
Faced with the above challenges, Ervin Industries was approached by a trailer manufacturer to identify an alternative to AlOx. Given the requirement for a corrosion-resistant abrasive, the logical suggestion was stainless steel grit.
THE CHOICE OF ABRASIVE
The two main aspects determining the choice of abrasive are (1) its ability to create a surface profile comparable to that created by AlOx, and (2) corrosion resistance. These aspects are, in turn, determined by the characteristics of the abrasive—its physical properties and chemistry. Stainless steel grit provides a rough surface profile instead of denting the surface like stainless steel shot. The stainless steel grit manufactured by Ervin is a contaminant-free, inert metal abrasive with a martensite/chrome carbide microstructure. Its hardness is at a minimum 57 HRC, giving it the power to etch. In terms of chemistry, it has a high degree of chromium (30%), in addition to 2% carbon, 4% silicon and 2% manganese.
The chromium content gives the abrasive its non-corrosive property. Parts blasted with stainless steel grit and tested in a 24-hour salt spray display no signs of corrosion. This means the fugitive grains of stainless steel grit, if and when left behind in the part intricacies, no longer pose the threat of corrosion and its transference onto automobiles and railcars that the frame might transport in its useful life.
Surface Roughness Comparison
Surface roughness achieved is a function of several factors such as the blast (air) pressure, hardness of the substrate, type of nozzle (venturi/straight bore), nozzle distance from the substrate (ideally 4 to 6 inches) and blast angle (90 degrees provides the maximum impact). A combination of these parameters will effectively determine and help alter the profile obtained. AlOx typically provides a surface roughness of 0.5 to 1.0 mil using a 120 mesh size sample. In comparison, a larger-sized sample, 60 mesh, could result in a surface roughness of 2.0 to 3.0 mil. Larger mesh sizes of AlOx could increase the profile to as high as 6 mil.The target of our case study was in the range of 1.5 to 3.5 mil.
As a general rule, the size of the stainless steel grit selected has to be one size smaller than the equivalent AlOx grade. Selection of the same size as AlOx will result in a surface roughness value that is at least 1 mil greater than obtained with AlOx. This is due to the stability of stainless steel grit and smaller percentage of smaller particles than AlOx. Due to its lower durability, AlOx is likely to have a relatively wider sieving range.
This prompted the end user to use SSG50 as their choice of abrasive to obtain surface roughness in the above range. In theory, the air pressure would also need to be reduced with the use of stainless steel grit. AlOx has a specific weight of 125 lbs/cft, compared to 280 lbs/cft for stainless steel grit. The end user continued to blast at the same air pressure—more on that discussion in the paragraphs that follow.
An offline lab test was conducted on three different sizes of AlOx and stainless steel grit to determine the consistency of surface finish over seven blast cycles, under two different air pressures.
Lower standard deviation is considered to be a process with better stability (values are closer to the mean). It was found that smaller abrasive sizes, AlOx and stainless steel grit, deviated the lowest from the value. Larger sizes, in both cases, showed high deviation, signifying reduced consistency. In general, larger sizes of AlOx (AlOx 30 and 46) showed greater standard deviation than their stainless steel grit counterparts. Further studies, such as a review of the surface topography, are required to ascertain uniformity of finish over a fixed area in order to determine suitability for higher precision applications such as aerospace and medical.
Figure 1: Variability of finish with AlOx
Figure 2: Variability of finish with AlOx
Air Pressure, a Critical Process Parameter
Common practice is to blast with as high a pressure as available in the plant. This could be in the range of 90 to 120 psi. The fact is that, at this pressure, AlOx faces rapid disintegration. Air pressure values below 50 psi are not even taken into consideration by most users, with the common complaint that “it takes too long” when blasted at this pressure. This statement can only be validated by reviewing all other factors at that time. For example, nozzle design—venturi or straight bore—will determine media dispersion and uniformity of surface profile. If a lower profile is desired and if lowering of pressure continues to provide a rougher profile than anticipated, the culprit might be the nozzle design.
The end user in our case study continued to blast at 90 psi, albeit with greater durability values of stainless steel grit than they experienced with AlOx. Let us assume that the application does allow for a reduction in air pressure— the economics of the operation will be significantly different.
For instance, consider the blast pressure dropping from 80 psi to 50 psi with stainless steel grit compared to AlOx. The difference in consumption of compressed air, by a 3/8-inch diameter nozzle, at 80 and 50 psi is about 50 cfm. This amount of compressed air will require a 10 HP compressor, which translates to a 7.5 kW savings per hour—not including breakdown costs and longevity of consumables such as nozzles and hoses.
Even at this pressure, due to the reduced breakdown of stainless steel grit, the end user started working in a blast environment that was significantly less cloudy (dusty), to the extent that their frequency of changing dust collector filter cartridges went from monthly to once a quarter.
Equipment Design Changes
The blast facility that blasts with stainless steel grit should have a robust media recovery system, as this media is highly recyclable. The operating costs can be kept in check because the media is 100% recoverable and reusable (after cleaning). The customer in our case study had a mechanical recovery system, which was agnostic to the type of media being reclaimed. Therefore, they had to make minimal modifications to adapt the system to stainless steel grit.
At the time of purchase, vacuum recovery systems are sold and designed for the media proposed for the project. If this media happens to be AlOx or glass bead discharged from a single nozzle, the blast machine will be supplied with a vacuum recovery system. This means that the exhaust fan located downstream to the machine will need to be re-evaluated for its capacity to move the normal amount of abrasive discharged from the nozzle(s). Most often, this could be an easy modification, such as a larger fan and/or motor sized to handle heavier steel media. The most important aspect of using stainless steel (or for that matter, any abrasive that has higher durability and recyclability) is that blasting is conducted in an enclosed environment with a functional recovery system.
Transmitted EnergyAn offline test was conducted with three different sizes of AlOx and stainless steel grit. These tests were carried out at two different air pressures, 30 psi and 50 psi. Though such pressures are notably lower than the 90 psi at which the end user was blasting, these values were selected to define this material property and gage its suitability for other applications—such as in aerospace prior to thermal spray.
The results that followed were interesting but not surprising. Given comparable cleaning properties, albeit with a smaller-size stainless grit, the transmitted energy was the same with both abrasive types. This was checked by using an Almen N strip (thickness = 0.031 inch) commonly used in shot peening applications. Transmitted energy was checked in terms of deflection of this strip when exposed to similar time cycles.
Figure 3: Transmitted energy with AlOx and stainless steel grit
ECONOMICS OF OPERATION
Several factors influence the economics of the blast operation. These factors are identified earlier as those affecting the surface roughness. Here we will evaluate the tangible aspects of using stainless grit, considering a blast system with a single blast nozzle.
Using conservative estimates, the cost of blasting with AlOx is twice that of stainless steel grit. The other aspects that haven’t been quantified above include:
Table 1: Operating Cost Analysis, AlOx and SS grit
Mineral abrasive such as garnet and AlOx continue to be popular choices. Is the switch to stainless steel grit for all applications? Probably not. As we discuss this, each of us is are aware of at least a dozen installations that blast in the open, in environments that cannot physically contain the abrasive, and at times without proper ventilation. These are the limitations, and the looming capital expense of an airblast room, even if it were just a freight container, and proper media recovery system, could be deterrents.
Most blast equipment companies have a cost-effective solution to this. DIY rooms and standard, plug-in recovery systems are not very expensive and within the reach of most finishing operations. Building an enclosure to contain and recover the blast media might sound daunting, but the payback will be quite rapid (in certain cases, less than a year).
For more information, visit ervinindustries.com.
Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.